Localising Barriers Theory

Government-Binding Parsing has become attractive in the last few years. A variety of systems have been designed in view of a correspondence as direct as possible with linguistic theory ([Johnson, 1989], [Pollard and Sag, 1991], [Kroch, 1989]). These approaches can be classified by their method of handling global constraints. Global constraints are syntactic in nature: They cover more than one projection. In contrast, local constraints can be checked inside a projection and, thus, lend themselves to a treatment in the lexicon. Conditions on features have been the subject of intensive study and viable logics have been proposed for them (see e.g. the CUF formalism [Dhrre and Eisele, 1991], [Dorna, 1992]). In this paper, we assume such a unification-based mechanism to take care of local conditions and focus on global constraints. One class of approaches to principlebased parsing (see [Pollard and Sag, 1991] for HPSG, [Kroch, 1989] for TAG) at tempts to reduce global conditions to local constraints and thus to make them accessible to treatment in a feature framework. This strategy has been pursued only at the expense of sacrificing the precise formulation of the theory and the definitory power stemming from it. The result has been a shift from the structural perspective assumed by GB theory to the object-oriented view taken by unification formalisms. The other class of approaches ([Johnson, 1989]) has allowed the full range of possible restrictions on trees and has incurred potential undecidability for its parsers. We take up a middle stance on t he matter in that we propose a separate logic for global constraints and posit that global constraints only work on ancestor lines (see 7).